1. Field of the Invention
The present invention relates to radio frequency or microwave circuits and antennas and more particularly to a method and apparatus for transferring radio frequency and phase control signals between a backplate support structure and monolithic microwave integrated circuits. The invention further relates to integration of monolithic via holes on the backside of monolithic integrated circuits with hybrid conductive vias in a backplate which incorporates material exhibiting high mechanical stability and thermal conductivity, and also supports phase control logic integrated circuits.
2. Related Technical Art
Communication and navigation systems, tactical and strategic sensors, and electronic warfare systems are some of the applications currently benefiting from the use of Monolithic Microwave Integrated Circuit (MMIC) technology. This technology employs high-volume, automated production and batch fabrication techniques to fabricate fully integrated circuits having arrays of antenna elements, associated power amplifiers, phase-shifters, and requisite mounting and interconnection structures on single wafers or substrates to reduce cost and imporve system reliability. In addition, the level of miniaturization and extremely accurate element positioning required for advanced signal processing applications are only achievable using maximum monolithic integration of the circuit functions.
Advanced phased-array applications generally dictate a very large number of antenna elements in the array to support high gain or large sweep angle requirements. In typical applications being developed, such as Extra High Frequency (EHF) 20 to 50 GHz antennas, a given array consists of from 3000 to 5000 antenna elements in a rectangular array. The face of each array is also covered with the associated amplifier and phase shifter elements which are interspersed between the antenna elements. In a typical phased array, the antenna element spacing is about one half of the desired free-space wavelength. At a frequency of 40 GHz, the wavelength is about 0.295 inches, which requires an array on the order of 8-10 inches on a side to accommodate the desired number of elements.
Unfortunately, the desired array sizes are typically ten to one hundred times larger than MMICs can be manufactured while maintaining reasonable yields and performance. Therefore, monolithic phased-element-arrays for use in extremely high frequency transmitters and receivers represent an application of MMIC technology that is also being developed using backplate technology. That is, an array of MMICs with their individual or sub-arrays of antenna elements, and associated integrated circuit components, are supported on a backplate to provide a rigid support and electronic signal transfer and interconnection structure.
Signals for controlling individual MMIC amplifier functions and phase shifter operations, as well as the desired radio frequency signals to be processed, must be routed to each MMIC in the array supported by the backplate, and each MMIC element in a sub-array. Most MMICs have electrical contact pads or connection points along their top edges, to allow for connection to very fine bond wires, ribbons, or miniature cables. The bond wires are used to connect the MMICs to various DC biasing or control signal sources. In the standard approach, small coaxial cables are employed to transfer RF type signals and prevent or reduce interference for adjacent antenna elements.
In conventional techniques, precision hand-work is required for connecting gold ribbon, bond wire, or coaxial cables to each contact pad. In addition, free volume or space is required to accommodate wires as they are fed around the edges or over the surface of each MMIC for connection to other apparatus. An alternative is to use large diameter passages extending through the MMIC which allow for the passage of small cables or wires through the MMIC for connection to other apparatus. Unfortunately, this undesirably consumes additional MMIC surface area and presents problems with element spacing.
The present MMIC interconnection structures are very labor intensive to construct and inspect. For large arrays having thousands of elements, the cost of labor often becomes prohibitive for all but advanced military applications. Even with modern automated assembly equipment, the construction time is very lengthy because the interconnection scheme is highly complex. Complexity and labor intensive fabrication also reduces functioning array or sub-array yield. At the same time, the large number of jumper type connections decreases array reliability by increasing damage due to handling and mechanical stress.
Current MMIC arrays also tend to be customized structures with variations in reliability and performance characteristics from array to array. Exact power requirements, channel cross-talk, and packaging problems vary from array to array. This lack of reproducibility and manufacturing consistency prevents wider application of MMIC arrays.
In addition to interconnection problems, many arrays must accommodate high power signals and temperature variations between -54.degree. C. to +71.degree. C., which also requires some form of thermal stabilization to counter material stress. Antenna arrays may also need to have sufficient physical strength to act as load bearing surfaces, such as in airplane skins, and match certain surface configurations.
What is needed is a method of producing large MMIC arrays, such as for phased-array antennas and the like, with reduced fabrication complexity and cost, and with increased manufacturing throughput and reliability. It is desirable to improve array-to-array reproducibility while reducing problems with array handling and interconnection. At the same time, any new method or design should also improve thermal stability.